The invention relates to gas turbine engines. More particularly, the invention relates to low pressure compressor sections of multi-spool turbofan engines.
A gas turbine engine may have one or more spools. Each spool may, for example, include the blades of associated compressor and turbine sections and a connecting shaft. Two- and three-spool engines are known. An exemplary two-spool engine included low and high speed spools. At cruise conditions, exemplary low spool speeds are 2000-7000 revolutions per minute (RPM) while exemplary high spool speeds are 9000-21000 RPM. The low spool includes low speed/pressure compressor and turbine (LPC and LPT) blades while the high spool includes high speed/pressure compressor and turbine (HPC and HPT) blades. A core flowpath through the engine may sequentially pass through the LPC, HPC, combustor, HPT, and LPT.
In a turbofan engine, a fan drives air along a bypass flowpath. In many engines, the fan may be mounted as part of the low speed spool and may be partially integrated with the LPC. In designing an engine, there may be a mismatch between desirable fan speed and desirable low spool speed. Particularly in high bypass turbofan engines it is advantageous that the fan speed be less than the low spool speed. In an exemplary intermediate speed compromise, the fan may be smaller than otherwise desired and the LPC and LPT may have a greater number of blade stages than otherwise desired. As an alternative, the low spool may drive the fan through a reduction gearing system (e.g., an epicyclic system). This permits the LPC and LPT to operate at the relatively high speeds at which they are efficient while the much larger diameter fan operates at the relatively lower speeds at which it is efficient.